Interpretive Summary: Russian thistle (common tumbleweed) is an alien weed that has invaded about 80 million acres in the western U.S. Tumbling plants disrupt automobile traffic, clog irrigation systems and pile up against buildings and fences. It is a major weed in dryland farming systems, especially spring wheat. The weed harbors insect pests that transmit curly top virus, which affects many important vegetable crops. The eriophyid mite, Aceria salsolae, is being evaluated as a prospective classical biological control agent of Russian thistle. A field experiment conducted in Italy showed that the mite did not attack nontarget plants and that it severely reduced growth of the weed. These results indicate that the mite will not damage nontarget plants and that it should significantly reduce the size and reproduction of the target weed. Release of this mite should significantly reduce infestations of the weed, which will reduce the economic costs and environmental pollution associated with conventional herbicide and tillage control methods.

Technical Abstract:
The eriophyid mite, Aceria salsolae attacks several species of invasive alien tumbleweeds, including Salsola tragus, S. collina, S. paulsenii and S. australis, in North America. Previous laboratory experiments to determine host specificity of the mite indicated that it could sometimes persist and multiply on some nontarget plants, including Bassia hyssopifolia and B. scoparia (=Kochia scoparia). These are both European plants whose geographic range overlaps that of the mite, but the mite has never been observed on them in the field. A field experiment was conducted in Italy to determine if the mite would infest and damage these plants under natural outdoor conditions. Our results indicate that this mite does not attain significant populations on these nontarget plants nor does it significantly damage them. Salsola tragus was heavily infested by mites, and plant size was negatively correlated to the level of infestation. Although S. kali plants were also infested, their size did not appear to be affected by the mites. The field results indicate that the nontarget plants that previously showed some suitability in the laboratory were not as suitable in the field. Combining these results with those of prior studies indicates that this mite is sufficiently host specific to pose no significant risk of damaging nontarget plants in North America.